1. The Field of the Invention
The present invention is in the field of hydrocracking high molecular weight hydrocarbon waxes into lower molecular weight, lower boiling point, higher quality materials. More particularly, the invention relates to a system and method for hydrocracking hydrocarbon waxes generated by a Fischer-Tropsch process that are contaminated with solid particulate impurities such as catalyst fines.
2. Related Technology
The conversion of fossil fuels such as coal, natural gas and petroleum coke to liquid hydrocarbon fuels and/or chemicals has been the subject of intensive research and development throughout the industrialized world for many years to provide a practical alternative to petroleum crude oil production and open-up the world's vast reserves of coal as a competitive source for essential hydrocarbons. Many processes have been developed for the direct or indirect catalytic hydrogenation of fossil fuels to yield liquid hydrocarbons. Some large pilot plants have been built and operated, and several commercial scale plants have been built for the conversion of coal to primarily liquid hydrocarbons. Of these plants, most were built by the German government during World War II. About half of them were built using the well-known Fischer-Tropsch process for converting synthesis gas to liquid hydrocarbons in contact with an iron catalyst. Such plants, operationally at least, worked well enough for war-time needs. Subsequently, the South African Government (SASOL, Ltd) built commercial size coal conversion plants to produce hydrocarbon fuels and chemicals which also were successfully based on indirect conversion using Fischer-Tropsch chemistry and iron catalysis.
From an operational point of view, the commercial liquefaction of coal or natural gas based on indirect Fischer-Tropsch (F-T) chemistry has been demonstrated to be an engineering success. However, true economic success has so far eluded the developers of direct or indirect coal or natural gas liquefaction processes, largely because of the historically low cost of crude oil as the competitive alternative. Nevertheless, there is now a genuine potential for indirect coal or natural gas liquefaction via Fisher-Tropsch (F-T) chemistry in view of more recent increases in crude oil price.
A known, practical method for preparing liquid hydrocarbons rich in valuable α-olefins is to convert a relatively low cost hydrocarbon material (e.g., coal, biomass or natural gas), to synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, by partial oxidation and/or steam reforming, which is followed by conversion of the synthesis gas to liquid hydrocarbons over a F-T catalyst (e.g., iron or cobalt). However, many catalysts used in the F-T process are especially fragile and break down easily in the F-T synthesis reactor into very fine particulates. In addition, a significant portion of the F-T synthesis products comprise high molecular weight, high boiling waxy hydrocarbons, which become mixed with the catalyst particles. These fine particles become dispersed throughout the waxy F-T product, and must typically be removed prior to hydrocracking the waxy F-T product because the solid particulates will otherwise cause plugging of downstream hydroprocessing reactors used to upgrade the waxy portion of the F-T products (i.e., fixed bed reactors having a stationary catalyst bed).
Hydrocracking the F-T waxy product portion to produce lower boiling point, more valuable products such as naphtha, diesel, and other light hydrocarbons is normally accomplished in a fixed bed reactors with stationary catalyst beds. In existing systems, hydrocracking and other hydroprocessing catalysts are arranged as a fixed or stationary bed within the reactor. The fixed bed may include a porous substrate having a very large surface area throughout which an active metal catalyst is dispersed. If catalyst fines (e.g., having an effective diameter less than about 200 microns) carried over from the F-T synthesis reactor are not suffiently separated from the wax before hydroprocessing, they will typically pile up within the interstitial spaces between the fixed bed of supported catalyst, thereby plugging the space between the supported catalyst where the liquid would normally flow. Extremely small fines can also plug the pores of the supported catalyst. The result is a drop in pressure, a loss of catalyst action, and a reduction in product yields. Deactivation of the fixed catalyst bed requires the reactor to be shut down for cleaning and catalyst replacement, which is extremely inconvenient, time consuming, and expensive.
While necessary with existing methods, separation of the solid particulates from the waxy product of the F-T process represents an added expense, and separation of the very fine particles from the wax can be extremely difficult. Costly and complicated separation processes, such as centrifuging or ultrafiltration, must be employed to effect removal of the very small catalyst particles so as to prevent plugging and deactivation of the downstream hydroprocessing equipment.
It would thus be a significant improvement in the art to provide a method and system for hydroprocessing the F-T wax products contaminated with solid particulates to produce more valuable lower molecular weight, lower boiling range products without requiring separation of the solid particulates from the F-T wax feedstock.